Discussion about EAD & END terminology

[Salvador Cuñat]

On Tue, Dec 10, 2013 at 08:51:45AM +0100, Henrik Brautaset Aronsen wrote:
> On Tue, Dec 10, 2013 at 12:15 AM, Salvador Cuñat 
> 
> 1) "EAD --> Equivalent Air Depth, considering N2 and He if diving trimix"
> 
> This is just wrong.  You are talking about Helium and Trimix in the context
> of EAD, which is weird.  EAD is mainly used to show the depth as if you
> where diving on air, when doing Nitrox dives.  That way I can use Air
> tables when diving with Nitrox.

As said, I'm not tec diver and when I dive nitrox, EAD, for me, is
exactly what you say. In fact, on nitrox the code performs this way.

But, when diving trimix, my understanding is that you need to take He
into account, as it's an inert gas, as inert as N2 is (with the obvius
differences).  The same way operates your example:

> The EAD (Equivalent Air Depth) at 57 meters is = ((1 - fO2) x ATA) /0.79
> = ((1 - 0.18) x 6.7) / 0.79 = 6.9 ATA = 59 meters

It only take out of the mix the fO2, so, on hypoxic mixes the sum of
fN2 and fHe are bigger than fN2 on air and then EAD > Depth.

Only tech divers dive trimix, and they are supossed to understand why
their EAD is bigger than their real depth. Recreational divers will get
exactly what they expect, as you say, the depth if they were diving air.

> 
> 2) "Equivalent Narcotic Depth, considering both N2 and O2 narcotics."
> 
> The description is better, but the calculation looks off somehow.  The role
> of END is to take into account the narcotic content of the gas, when doing
> Trimix dives: (pO2 + pN2) == (1 - pHe).  It is used to show how narcotic my
> trimix gas at a given depth is, compared to when diving on air.  If you
> look at my previous calculation:
> 
>    The END (Equivalent Narcotic Depth) at 57 meters is (1 - fHe) x ATA = (1
> - 0.45) x 6.7 = 3.7 ATA = 27 meters
> 
> That means that when diving with 18/45 to 57 meters, I will not get more
> narked than if I was diving with air to 27 meters.  Good stuff.
> 

It seems to me that we totally agree in the concepts and there is only a
problem involving the maths in the code and my bad explanation in my
worse english.

I tried to change Jan's code as minimum as posible but I recon it's
difficult to understand or even to follow in this part at least.

I'm sending another patch with better explanation (I expect) and simpler
maths.

Please take a look on it.

Regards.

Salva.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: Adecuate-EAD-and-END-to-their-more-standarized-meanings.patch
Type: text/x-diff
Size: 2057 bytes
Desc: not available
URL: <http://lists.hohndel.org/pipermail/subsurface/attachments/20131210/2fd15ab0/attachment-0001.patch>